Frequency converters typically have a DC voltage intermediate circuit which stores DC voltage for use for the inverter part. The inverter of a frequency converter is typically used for producing controlled alternating voltage to a load from the voltage of the intermediate circuit. The DC voltage to the intermediate circuit is rectified typically from AC mains voltage by using a rectifier bridge.
The DC voltage intermediate circuit or DC bus contains one or multiple capacitors for storing and smoothing the voltage. The capacitance of the intermediate circuit capacitors is large and when the frequency converter is taken into use, these capacitors have to be charged before any control operations can be carried out.
The capacitors of the intermediate voltage circuit are charged from the supplying mains voltage. The charging current has to be limited to a suitable level since otherwise the large current would harm the electrical components of the rectifier or cause erroneous operations of the protective components, such as fuses and like. The current limitation is typically carried out either by using a charging resistor or by controlling the controllable switches of the rectifier.
The use of a charging resistor is a simple and reliable method for charging the intermediate circuit. Charging with a charging resistor requires additional components which are expensive especially when the power level is high.
One type of rectifier commonly used also in frequency converters is a half-controlled rectifier bridge. The half-controlled bridge consists of multiple series connections of thyristors and diodes, and usually the thyristors are the upper components and thus their cathodes are connected to the positive rail of the DC intermediate circuit.
Once the intermediate circuit capacitor has been charged to a voltage that is near the value of the rectified mains voltage, the charging phase is over and the thyristors in the rectifier are usually operated as diodes. This means that the thyristors are controlled to a conducting state as soon as the anode-to-cathode voltage is positive, and the thyristor can be triggered to a conducting state. This operation of thyristors in diode mode, i.e. at full phase angle, ensures maximal voltage to the DC intermediate circuit.
It is widely known that in control methods where also the diode mode relies on phase angle control, there is a compromise between reliability and dynamic response. This may cause malfunctions in line voltage dips and other rapid changes of the line voltage.
It would be desirable to obtain a phase angle control circuit that is simple in design and enables fast and reliable operation in diode mode for charging the intermediate circuit capacitor. Further it would be desirable to be able to resize the control circuit topology easily to all power levels.